The present invention relates to an implantable device which senses abnormal heart beat rates and delivers stimulating electrical pulses to the heart in order to correct such abnormalities. Particularly, the present invention relates to a pacer/cardioverter that is capable of detecting arrhythmias requiring pacing and also ventricular fibrillation, and of providing the appropriate treatments, and a corresponding method of such detecting and treating a heart.
It is well known that the heart can be monitored by sensing the electrical activity thereof. Many processing schemes have been devised to determine the condition of the heart and to particularly determine whether the heart is beating at an abnormally slow rate (bradycardia), a normal rate (normal sinus rhythm), an abnormally fast rate (tachycardia), a generally chaotic fast rate (ventricular fibrillation), or has substantially ceased to beat (asystole).
The electrical activity of the heart can be sensed and the resultant signal pre-processed (for example, by pre-amplification, filtering, etc.), and then digitized in some fashion. The digitized signal can further be processed to specifically diagnose the condition of the heart. These operations can occur in an implantable device. Based upon the diagnosis, stimulating pulses are applied to the heart from the implantable device. The stimulating pulses may consist of pacing pulses, a low level electrical shock pulse, or a high level electrical shock pulse. The low and high level shock pulses are called herein "cardioverting pulses" which are commonly in the neighborhood of one joule of energy or more in contrast to pacing pulses which are in the microjoule energy range.
In some situations, the electrical activity of the heart during ventricular fibrillation is at a very low amplitude level. If the implantable device tests whether the signal obtained from the heart, herein called a "cardiac signal", exceeds a threshold level, the device may diagnose a heart condition as asystole (no heartbeat) or bradycardia (slow heartbeat) and issue pacing pulses when, in fact, the heart is in ventricular fibrillation (VF) because the low level electrical activity indicative of VF is insufficient to trigger the threshold detection circuitry of the implantable device. Such pacing pulses could be detected by the sensing circuitry and further interfere with the recognition of the life-threatening ventricular fibrillation.